We have at Brown University a microspectrofluorometry and time lapse photography facility. The addition of a flow cytometer capable of cell sorting and an image analysis system capable of handling both normal and low light intensity images will greatly expand the range of possible research projects requiring quantitative optical methods applied at the level of the single cell or cell organelle. The necessity of the flow cytometer and image analysis system to current research of 23 investigators is described. The flow cytometer will be used to sort preneoplastic from normal cells, as well as different normal cell types before or after overt differentiation, to isolate a particular chromosome, and to analyze mixed cell populations characterized by different enzyme activities, antigens, or size. The image analysis system is needed to quantify: the number of plasmids per E. coli cell, the characteristics of Drosophila mutants, the behavior of platelets in the circulatory system, and sizes and numbers of subcellular structures, including those marked by bound colloidal gold in electron micrographs. The projects include both techniques previously used and new ones with the potential for broad application. The data obtained will add a new dimension to ongoing research and increase very significantly its precision.